This invention relates to dental powder composites and more particularly to such composites adapted to be combined with mercury to form dental amalgam and to such amalgams.
Commercial dental alloys are generally supplied in powder form or in loosely compressed tablets of alloy powder material. The powder or tablet form is combined with mercury (trituration procedure) to form the amalgam which is used for filling of dental cavities. In general only sufficient mercury is used to provide the required plasticity to permit packing of the amalgam into the cavity. The plasticity of the amalgam is dependent upon several factors including the composition of the alloy, the particle size of the powders and trituration mode (time and energy input). For most commercial dental alloys the mercury to alloy weight ratios recommended for trituration fall in the range of 1:1 to 1.6:1.
It has now been well established that the mechanical properties of the dental amalgam deteriorate as the residual mercury content increases. Mahler and Van Eysden in the Journal of Dental Research, 1969, Vol. 48, No. 4, page 501, have shown that the properties of dynamic creep, static creep, ADA flow, 1 hour and 7 day compressive strengths as well as 7 day tensile strength of the amalgam are all significantly poorer when the residual mercury content of the amalgam increases from 48% to 53%. Further, Mateer and Reitz in the Journal of Dental Research, 1970, Vol. 49, No. 2, page 399, have shown that the principal mechanism in amalgam corrosion is the breakdown of the tin-mercury phase (gamma-2), the formation of such phase being increased in the amalgam by the presence of excess liquid mercury during trituration as shown by Gaylor in the Journal of the British Dental Association, 1936, Vol. 60, page 11. These various investigations clearly show that to attain optimum serviceability from amalgam restorations a minimum effective amount of mercury should be employed for trituration in developing the desired plasticity. On the other hand, it is also necessary that sufficient mercury be used to provide the desired plasticity; total elimination of mercury as taught by Baum, U.S. Pat. No. 3,495,972 is seen to be undesirable because of the lengthening of the setting time which results.
It is recognized that the addition of many metals, including indium, to dental amalgam has been known in the prior art, as shown for example in Gray, U.S. Pat. No. 1,959,668; Gray, U.S. Pat. No. 1,963,085; Beldham, U.S. Pat. No. 3,554,738; and Muhler, U.S. Pat. No. 3,676,112. But all these patents teach either the alloying of the indium with the silver-tin alloy or the combination of indium with mercury prior to the amalgamation of the silver-tin alloy. None teaches combination of unalloyed indium powder with the silver-tin alloy prior to amalgamation thereof. Even my own U.S. Pat. No. 3,305,356, which teaches that indium may be alloyed with silver and other metals to form dental alloy, does not teach the addition of unalloyed indium powder to the silver-tin alloy.
It has now been discovered that the addition of discrete particles of unalloyed indium to silver-tin alloy prior to amalgamation substantially reduces the deleterious gamma-2 phase and allows trituration with less mercury than known in the prior art, thus resulting in a mechanically stronger dental amalgam.
An object, therefore, of this invention is to provide a dental alloy powder to attain improved properties of the formed dental amalgam.
A further object of this invention is to provide a dental alloy powder which substantially decreases the amount of mercury required for amalgamation and hence reduces the residual mercury content of the amalgam.